1. Field of the Invention
The present invention relates to radiation dosimetry systems and networks of radiation detectors and, particularly, to accurate calculation of the equivalent absorbed dose due to a radiation exposure event.
2. Background of the Invention
Occupational radiation exposure events can occur in healthcare, the oil and gas industry, the military and other industrial settings where the use of materials or devices that emit ionizing radiation can result in accidental or occupationally unavoidable exposure events.
Emergency radiation exposure events can occur when a Radiological Dispersal Device (RDD), Improvised Nuclear Device (IND), or another source of radioactive material is released and contaminates a given area.
Radiation dosimetry programs have been developed to monitor and protect workers who might be exposed to radiation. The personal dose equivalent, measured using a radiation dosimeter, is commonly used to monitor radiation dose to an individual. The accurate and reliable measurement of the personal dose equivalent from a radiation exposure event is a key component of radiation dosimetry. The personal dose equivalent is typically measured over a wide range of energies and from different radiation sources, which might include x-rays, gamma rays, alpha particles, beta particles and neutrons. In order to accurately estimate the dose from different radiation sources, many personal dosimeters incorporate an array of detector elements, each with varying types of radiation filtration materials, and use a dose calculation algorithm to accurately calculate the personal dose equivalent from a numerical combination of the responses from each detector element.
More recently, emergency management plans have been developed to enable the safe and timely response to emergency radiation events. An important aspect of any emergency management plan following a radiation event is to ensure the safety of fire, police and other emergency response personnel (“First Responders”), health-care workers, and citizens that might be exposed to radiation resulting from the radiological or nuclear device. Radiation exposure of first responders and health-care workers is often, at least partially, monitored using traditional radiation detecting devices, however, monitoring the exposure of potentially tens of thousands of citizens presents a more difficult problem.
Furthermore, after the removable contamination has been eliminated, there may be a need for ongoing external personal dosimetry monitoring for individual First Responders, healthcare workers, and members of the public. Site restoration could be a lengthy project and, to minimize disruption to society, it may be necessary to allow inhabitants to have access to certain areas before cleanup is complete. For example, allowing citizens to pass through transit centers, thoroughfares, or certain areas of buildings would facilitate government operations, commerce, uniting of families, routine medical treatments, etc. As an individual moves through a contaminated area, it would be valuable to know the dose and time of exposure at each location visited. Such dose measurements could reduce reliance on model-based estimates of dose, and avoid unnecessary area restrictions by providing a geographic map of the dynamic dose distribution reconstructed from a large number of dosimeters collecting dose event data over the potentially still-contaminated area. Unlike cleanup at decommissioned facilities where the public could be excluded with little cost to society, in an urban environment, time is of the essence and the cost of exclusion may be greater than the benefit avoiding exposure to a relatively low radiation dose. After cleanup, personal dosimetry could boost public confidence that their personal dose is below acceptable thresholds, and that the final cleanup was effective.
Several radiation measurement technologies currently exist including TLD dosimeters, OSL dosimeters, electronic dosimeters, quartz or carbon fiber electrets, and other solid-state radiation measurement devices.
Thermoluminescent Dosimeter (TLD) badges are personal monitoring devices using a special material (i.e. lithium fluoride) that retains deposited energy from radiation. TLD badges are read using heat, which causes the TLD material to emit light that is detected by a TLD reader (calibrated to provide a proportional electric current). Significant disadvantages of TLD badges are that the signal of the device is erased or zeroed out during reading, and the dosimeters must be returned to a processing laboratory for reading, and substantial time is required to obtain the reading.
Optically Stimulated Luminescence (OSL) badges use an optically stimulated luminescent material (OSLM) (i.e., aluminum oxide) to retain radiation energy. Tiny crystal traps within the OSL material trap and store energy from radiation exposure. The amount of exposure is determined by illuminating the crystal traps with a stimulating light of one color (i.e., green) and measuring the amount of emitted light of another color (i.e., blue). Alternatively, pulsed light stimulation can be used to differentiate between the stimulation and emission light [e.g., see U.S. Pat. Nos. 5,892,234 and 5,962,857]. Unlike TLD systems, OSL systems provide a readout in only a few seconds and, because only a very small fraction of the exposure signal is depleted during readout, the dosimeters can be readout multiple times. OSL dosimeters can be read in the field using small, field-transportable readers, however, the readers are still too large, slow and expensive to allow individual, real-time readings in the field. In currently-existing OSL dosimetry programs for reporting the dose of record, the dosimeters must be returned to a processing laboratory for readout.
Optically Stimulated Luminescence (OSL) badges use an optically stimulated luminescent material (OSLM) (i.e., aluminum oxide) to retain radiation energy. Tiny crystal traps within the OSL material trap and store energy from radiation exposure. The amount of exposure is determined by illuminating the crystal traps with a stimulating light of one color (i.e., green) and measuring the amount of emitted light of another color (i.e., blue). Alternatively, pulsed light stimulation can be used to differentiate between the stimulation and emission light [e.g., see U.S. Pat. Nos. 5,892,234 and 5,962,857]. Unlike TLD systems, OSL systems provide a readout in only a few seconds and, because only a very small fraction of the exposure signal is depleted during readout, the dosimeters can be readout multiple times. OSL dosimeters can be read in the field using small, field-transportable readers, however, the readers are still too large, slow and expensive to allow individual, real-time readings in the field. In currently-existing OSL dosimetry programs for reporting the dose of record, the dosimeters must be returned to a processing laboratory for readout. For more information on OSL materials and systems, see, U.S. Pat. No. 5,731,590 issued to Miller; U.S. Pat. No. 6,846,434 issued to Akselrod; U.S. Pat. No. 6,198,108 issued to Schwietzer et al.; U.S. Pat. No. 6,127,685 issued to Yoder et al.; U.S. patent application Ser. No. 10/768,094 filed by Akselrod et al.; all of which are hereby incorporated by reference in their entireties. See also, Optically Stimulated Luminescence Dosimetry, Lars Botter-Jensen et al., Elesevier, 2003; Klemic, G., Bailey, P., Miller, K., Monetti, M. External radiation dosimetry in the aftermath of radiological terrorist event, Rad. Prot. Dosim, in press; Akslerod, M. S., Kortov, V. S., and Gorelova, E. A., Preparation and properties of Al.sub.2O.sub.3:C. Radiat. Prot Dosim 47, 159-164 (1993); and Akselrod, M. S., Lucas, A. C., Polf, J. C., McKeever, S. W. S. Optically stimulated luminescence of Al.sub.2O.sub.3:C. Radiation Measurements, 29, (3-4), 391-399 (1998), all of which are incorporated by reference in their entireties.
Solid State Sensors use solid-phase materials such as semiconductors to quantify radiation interaction through the collection of charge in the semiconductor media. As the radiation particle travels through the semiconductor media electron-hole pairs are generated along the particle path. The motion of the electron-hole pair in an applied electric field generates the basic electrical signal from the detector. There are two main categories of solid state sensors, active and passive. Active sensors often use a semiconductor that is biased by an externally powered electric field that requires constant power. The active sensors generate electric pulses for each radioactive particle striking the sensor. These pulses must be continuously counted to record the correct radiation dose. A loss of power means no dose is measured. Active solid state sensors are typically made from silicon and other semiconductors. Passive solid state sensors utilize an on device charged medium that maintains the electric field necessary to separate the electron-hole pairs without drawing external power. Passive solid state dosimeters often use what is called a floating gate where the gate is embedded within the detection medium so it electronically isolated. The floating gate is charged and provides the electric field for charge separation (e.g., see U.S. Pat. No. 6,172,368 issued to Tarr). The medium above the floating gate is typically an insulator such as silicon oxide however it can also be a sealed gas chamber (e.g., see U.S. Pat. No. 5,739,541 issued to Kahilainen). Passive Solid state electronic detectors offer a means of monitoring radiation that are compatible the present invention.
Electronic dosimeters are battery powered, and typically incorporate a digital display or other visual, audio or vibration alarming capability. These instruments often provide real-time dose rate information to the wearer. For routine occupational radiation settings in the U.S. electronic dosimeters are mostly, but not strictly, used for access control and not for dose of record. A number of cities and states issue electronic dosimeters to HAZMAT teams as part of their emergency response plans. There are presently tens of thousands of electronic dosimeters deployed, for example, for homeland security purposes; however, electronic dosimeters are impractical for widespread use dosimeters due to their high cost.
Quartz or carbon fiber electrets are cylindrical electroscopes where the dose is read by holding it up to the light and viewing the location of the fiber on a scale through an eyepiece at one end. A manually powered charger is needed to zero the dosimeter. The quartz fiber electret is an important element of many state emergency plans. For example, some plans call for emergency responders to be issued a quartz fiber electret along with a card for recording the reading every 30 minutes, as well as a cumulative dosimetry badge or wallet card. While they are specified for use in nuclear power plant emergencies, the NRC does not require them to be NVLAP accredited, only that they be calibrated periodically.
Existing passive personal radiation monitoring devices do not provide immediate access to recorded dose measurements, while active devices typically consume sufficient power to require regular recharging. No existing devices measure the complete “radiation event.”
In general, a need exists for “event detection” devices, e.g., radiation dosimeters or other detection devices, with the following characteristics: (1) small and easily carried or mounted to fixed structures or mobile transports; (2) capable of measuring a dose event, including the measured amplitude or intensity of the event, time of the event, location of the event, ambient temperature, motion of the detector and proximity to other detectors; (3) accurate calculation of the dose, e.g., the Personal Dose Equivalent, over a wide dose range, wide energy range, and large angles of incidence; (4) ability to display the measured dose event on the detector, or using a personal mobile device, in order to alert the User to anomalous events, and in order to transmit the measured dose over private and public networks to a dose event repository; (5) ability to track and report dose events in the field over extended periods of time without replacing or externally charging the power source; (6) ability to map the distribution of dose over a geographic area, to identify anomalous dose distributions, to dynamically track sources and to alert Authorized Personnel of anomalous dose events.
The personal dose equivalent, measured using a radiation dosimeter, is the most commonly used metric of radiation dose to an individual. The accurate and reliable measurement of the personal dose equivalent is a key component of radiation dosimetry. The personal dose equivalent is typically measured over a wide range of energies and from different radiation sources, which might include x-rays, gamma rays, alpha particles, beta particles and neutrons. In order to accurately estimate the dose from different radiation sources, many personal dosimeters incorporate an array of detector elements, each with varying types of radiation filtration materials, and use a dose calculation algorithm to accurately calculate the personal dose equivalent from a numerical combination of the responses from each detector element.
In general, a need exists for small, low-cost, self-contained and field-readable radiation dosimeters that provide an accurate calculation of the Personal Dose Equivalent over a wide dose range, wide energy range, and over large angles of incidence.